btree index and max()

This issue applies to postgresql 6.5.3 & 7.0

Say I have a table 'FACTURAS' (~400k rows), with
a 'RID' field, which is indexed with an BTREE index.

If I want to get the max(rid), the index is not
used:

=> explain select max(rid) from facturas;
NOTICE: QUERY PLAN:

Aggregate (cost=21139.66 rows=342414 width=4)
-> Seq Scan on facturas (cost=21139.66 rows=342414 width=4)

(yes, I run 'vacuum analyze').

I understand that the query planner cannot be so clever
to grasp that this particular function (max or min)
might be evaluated by just travelling the BTREE index.
Am I correct?

If I modify the query with a dummy restriction:

=> explain select max(rid) from facturas where rid>0;
NOTICE: QUERY PLAN:

Aggregate (cost=9582.90 rows=114139 width=4)
-> Index Scan using facturas_rid_key on facturas (cost=9582.90 rows=114139
width=4)

... the index is used, but only to get the restricted set of rows,
not to evaluate the maximum. Hence, the performance the same.

Cheers

Hernan Gonzalez
Buenos Aires, Argentina